This invention pertains to airfoils with control surfaces, and more particularly to biasing means to positively position the control surface in a predetermined position relative to the airfoil in the absence of an overcoming actuator force.
Typically, in airfoil designs having a trailing control surface, e.g. a wing and aileron, it is necessary to provide means to restrain the control surface in the event the means used to actuate the surface fails in a fashion which allows the surface to rotate freely about its hinge line. Some retention means is necessary to stabilize the control surface allowing air loads to move the surface to the faired position relative to the airfoil so as to permit limited maneuvering and landing of the aircraft, after the failure, by use of the remaining control surfaces. Classically, mass was added at the proper location on the control surface so as to behave as a counterbalance, stabilizing the surface in a neutral or faired position. While the counterbalance technique added weight it was advantageous to prevent flutter and had the obvious advantage of minimizing the effect on the control stick forces as the only variables were aerodynamic load and the rate of acceleration of the control surface. Since the rate of acceleration of the control surface was a function of the rate the pilot moved the control stick, increased load as a result of increased acceleration rates were desirable. Had a spring been used as a baising means the stick force would have varied with stick displacement as a result of the springs. The counterbalance technique worked well with manually actuated control surfaces through direct drive cable systems or power boosted systems with conventional wing design as opposed to current advanced technology wing design. Basically, any boost system, whether aerodynamic boost or a boost actuator, involves, in part, a manual system.
Introduction of the fully powered control surface changed things somewhat. The above noted requirements essentially remain the same except the full powered system provides adequate muscle to drive the control surface and the load characteristics are not important as they are not felt by the pilot. This is true whether the actuator is driven by a cable which positions a valve or an electrical signal is carried on a wire to the actuator. Prior art full powered control systems employed a three actuator system where two actuators were redundant. The redundancy was required for safety. For example a failure that demands a hard-over control surface, like a valve jam, is passive in nature and can go undetected for long periods of time in the three actuator system. A hard-over command on the first actuator requires a similar hard-over command in the opposite direction to neutralize the force of the first actuator. Since these two forces are equal and opposite, if there were only two actuators, the surface would return to a neutral or faired position only if there was a restoring force sufficient to increase pressure in the jammed actuator above its relief valve setting. To remedy this problem a third actuator was employed, and since the first and second actuators offset each other, the third actuator was required to overcome the relief valve. The need for three actuators is really established by the fact that hardover jam failures cannot be tolerated in some control surfaces in certain phases of the flight envelope.
The advent of the advanced technology wing, which improves aerodynamic efficiency, moved the center of aerodynamic pressure aft on the airfoil as compared to that of the conventional airfoil design. Again, referring to the wing and aileron, this aft loading increases the aerodynamic forces acting on the control surface requiring greater forces to neutralize the surface when the acuator power is lost. Since there are practical limits on the length of the moment arm and structural limits, particularly at the wing tip, this generally means increased weight to balance the aerodynamic loads.
Another associated but not directly related problem is flutter of the control surface. In a true flutter condition the control surface is excited at its natural frequency and there is insufficient dampening to attenuate the vibration and the surface will continue to oscillate at greater amplitudes until failure occurs. Typically this happens as a result of a complex coupling of the control surface with another structure, such as the wing. Counterbalancing the control surface with weight produces an overbalancing moment which increases as the motion of the wing or main airfoil increases and acts in the opposite direction, i.e. out of phase, also, the overbalancing moment is directly proportional to the acceleration which is not true of the spring, probably the worst case occurs when there is some float in the control surface. This float could be supplied by sufficient backlash at the hinge line but more typically it occurs when there is a mechanical failure as discussed above, where the surface is free to float about its hinge line. If the surface is properly counterbalanced it will take care of the first problem, discussed above, which provides for stabilizing the surface, allowing the remaining controls to fly the airplane. However, in this arrangement, the surface is still free to float and may flutter if there is an adquate coupling of the structures and inadequate dampening.
The control surface must have a natural frequency sufficiently removed from the main airfoil to prevent coupling. The object of this invention is to make the main airfoil and the control surface act as a single unit by providing a stiff biasing means between the two surfaces.
In summary, prior art airfoils having trailing control surfaces were designed to accommodate a failure which permits the control surface to float by adding counterbalance weights to prevent the control surface from fluttering. The only resistance to the flutter is provided by the moment of inertia of the control surface unless fluid dampeners are added. Alternatively, in full power systems, it requires three actuators, two of which are redundant, to provide adequate safety and the system relies on the actuators to lock the system to prevent flutter of the control surface.
It is an important object of the present invention to provide a positive mechanical bias to the control surface which will return the control surface to a neutral or faired position, relative to the airfoil, in the event that all actuator power is lost.
Another important object of the present invention is to concurrently provide a clamping means at this neutral position which does not allow any float or freedom of rotation of the control surface to avoid a flutter conducive condition by making the main airfoil and the control surface behave as a single structure to avoid dynamic coupling.
A further object of this invention is to provide a full powered control surface employing two actuators, one for redundancy, along with the surface hold down mechanism of this invention which will accommodate an error which provides a hard-over command signal to one actuator which can be overcome by the second actuator as it is assisted by the force of the biasing means.
Another object of this invention is to provide a surface holdown mechanism which is entirely mechanical, highly reliable and provides a system with a detectable failure mode. Yet it is lighter, less complicated, cheaper, and more reliable than the methods of the prior art.